In patent ES2265760 metal inserts are housed and secured axially on the walls of the longitudinal resistant structure (blade beam). The housings are machined and the insert connection is secured with an adhesive. A cavity (16) is machined on a solid laminate (12) of carbon fibers (13) and glass fibers (14) and an insert (10) is inserted. The insert head can be of two types: metal U-shaped parts for traction bolts and shear connection element. These heads conceived for bolting insert pairs are large and heavy, since they include a housing for enabling the connection of nuts for pre-tensioning bolts, penalizing the connection in cost and extra weight.
In patent ES2369442, the inserts are embedded and inserted (first machining them as in the previous case) into the composite material constituting the blade. The insert body has a cylinder or cone shape. There are two types according to their placement:                Machined insert: A hole is machined into an already cured lamination, into which an insert, head (2) and body (3) are inserted, and affixed with adhesive (4). The lamination is made on a continuous thickness throughout the width of the section. A disadvantage is that the lamination is very thick and has a high cost in structurally unnecessary material, adding to the total weight of the module.        Embedded insert: The insert is laminated to add a reinforcement part (5) during the lamination process. The same laminate resin bonds the insert to the material after curing. A separator (6) is also added between inserts. A disadvantage is that unlike a machining process, this manual process is not easily automated and thus highly susceptible to environmental conditions and skill level of the workers. Moreover, to attain an acceptable load capacity, an additional part (6) is incorporated. This part must also be affixed onto the metal insert before lamination. Thus, in order to eliminate the disadvantage of extra mass from the option above (machined insert), other clear disadvantages are added such as the susceptibility of the process and the elevated extra cost arising from the part and additional affixing process.        
Furthermore, the tightening of joint elements used in these inserts is a complex process that either requires an external support point or is done by opposing pairs to utilize the action/reaction balance of each pair, thus requiring no external support. The connection process solution differs in the mentioned patents but always has a connection element: Patent ES2265760 uses a bolt and two nuts between opposing heads while patent ES2369442 uses two connections of a singe bolt and nut with different peculiarities (left-hand thread on insert, right-hand thread on nut and machined head for wrench fastening). In both cases, the two pairs are compensated with each other (the two-nut pair or the bolt-nut pair) and the extra support point is avoided, but in any case there is a need for a preferentially rectangular, heavy and expensive insert head enabling the housing of the mentioned nuts.
Alternatives for pre-tensioning bolts are disclosed herein. When pre-tensioning by torque, the bolts are compensated with the adjacent connection element. When pre-tensioning by pulling, the modules are separated to apply the necessary tension to the joint on all the bolts at the same time, thus eliminating the need for heads between modules and reducing the connection parts to a minimum expression and thus attaining a significant advantage in mass and cost compared with current state of the art. In both cases, all the bolts are pre-tensioned simultaneously by applying torque or pulling, which entails a substantial novelty regarding the state of the art.